What lies on the lower southern slopes of Knocknadobar mountain, overlooking the Ferta river and its estuary, is something rather more deliberate than a natural hollow: a souterrain, an underground passage constructed by hand from drystone walling and roofed with large flat slabs, of a type built across early medieval Ireland as a place of refuge, storage, or concealment.

The entrance is modest to the point of severity, a lintelled opening measuring roughly 80 centimetres by 45 centimetres, set into the southern face of the western end of a low earthen bank running east to west for about fifteen metres. Crouching through it, you would find yourself at the eastern end of a passage some four and a half metres long, sloping gradually downward and aligned along the same east-west axis. The walls are drystone, the roof a sequence of heavy overlapping slabs. The western end is now blocked by collapse, so the full original extent of the structure, if it ever continued further, can only be guessed at. The relationship between the passage and the bank above it suggests careful planning; the bank may have served to conceal or protect the entrance, though whether the two features were constructed together or at different periods is not recorded.